Using stereolithography tools for injection moulding: research into tensile tool failure and unexpected benefits of the process

The use of stereolithography (SL) parts as injection moulding tools offers many advantages over traditional tool making approaches. In particular, the time required to convert a computer aided design (CAD) file to the final t ool is dramatically reduced as may be the costs in creating the tool. Howev er, the process has been perceived to have a number of drawbacks which are, in the main, associated with the poor thermal and mechanical properties of the SL resins. Research was performed which involved the production and use of an SL tool to injection mould a series of parts in polypropylene. Measurements were ma de of the forces required to eject parts from the tool in order to assess t he possibility of tensile tool failure during part ejection. The measured f orces were compared with predicted values which had been calculated using p reviously devised equations. In addition, temperatures were recorded throug hout the injection moulding cycle to assess the tool's strength at differen t times. Surface roughness measurements were also taken to characterize the ejection process more closely and observe any changes to surface roughness caused by moulding parts. A finite element analysis was made of the heat t ransfer in the tool to help with prediction of ejection forces. Measurements of ejection forces and heat transfer indicated that, contrary to existing recommendations, part ejection should be performed shortly afte r injection. Accurate predictions of ejection force were shown to be unatta inable given the difficulty in finding accurate values for the variables us ed in the force prediction equations. Measurements of heat transfer coupled with surface roughness suggested that the low thermal properties of SL too ls actually work in favour of the process rather than against it. This conc eptual about-turn regarding the role of thermal properties in SL injection moulding tools indicates that further benefits may be derived from the new technology.